Mystery or mistery

MYSTERY or MISTERY. This word is said to be derived from the French mestier now written metier, a trade. In law it signifies a trade, art, or occupation. 2 Inst. 668.
     2. Masters frequently bind themselves in the indentures with their apprentices to teach them their art, trade, and mystery. Vide 2 Hawk. c. 23, s. 11.

A Law Dictionary, Adapted to the Constitution and Laws of the United States. By John Bouvier. Published 1856.